There are a number of in situ techniques for recovering hydrocarbons, such as heavy oil and bitumen, from subsurface reservoirs. Thermal in situ recovery techniques often involve the injection of a heating fluid, such as steam, in order to heat and thereby reduce the viscosity of the hydrocarbons to facilitate recovery.
One technique, called Steam-Assisted Gravity Drainage (SAGD), has become a widespread process for recovering heavy oil and bitumen particularly in the oil sands of northern Alberta. The SAGD process involves well pairs, each pair having two horizontal wells drilled in the reservoir and aligned in spaced relation one on top of the other. The upper horizontal well is a steam injection well and the lower horizontal well is a production well.
Another technique, called Cyclic Steam Stimulation (CSS), is used to cyclically inject steam and then produce hydrocarbons from the same well.
Numerous wells or well pairs are usually provided in groups extending from central pads for hundreds of meters often in parallel relation to one another in order to recover hydrocarbons from a reservoir.
For such thermal in situ recovery operations utilizing steam injection, a steam chamber is formed and tends to grow upward and outward within the reservoir, heating the bitumen or heavy hydrocarbons sufficiently to reduce the viscosity and allow the hydrocarbons and condensed water to flow downward toward the production well. Over time, the steam chambers expand and can coalesce. Eventually, the economics of hydrocarbon recovery begin to decline, for example as the steam-to-oil ratio (SOR) increases.
Despite the reduced economic viability of mature thermal in situ recovery operations, the reservoir often includes a significant amount of unrecovered hydrocarbons as well as a peripheral region around the steam chamber that was heated due to the presence of the steam. The heated bitumen or heavy hydrocarbons in these peripheral regions are valuable, but there are challenges related to the efficient recovery of these hydrocarbons.
In some instances, infill wells have been provided in between existing SAGD well pairs in an attempt to recover some of the unrecovered hydrocarbons located in between SAGD well pairs. However, infill wells also leave hydrocarbons in the reservoir.